Conventional electrically driven vehicles, such as an electric automobile or hybrid automobile using an electric motor for traveling, are equipped with two types of storage battery, namely, a high-voltage battery (drive battery) storing high-voltage power for driving the electric motor, and a low-voltage battery (auxiliary battery) storing low-voltage power for driving auxiliary equipment of the vehicle, such as lamps. The drive battery is charged with electric power supplied from an exterior charging device situated outside the vehicle. The auxiliary battery is charged as it is supplied with electric power from the drive battery via a voltage converter (DC-DC converter) for converting high-voltage power to low-voltage power.
The auxiliary battery generally comprises a lead-acid storage battery and generates heat during charging. Also, as the ambient temperature or the temperature of the auxiliary battery itself rises, a highest allowable voltage at and below which the auxiliary battery can be charged lowers, and if charging is performed at a voltage higher than the highest allowable voltage, heat generation is furthered by the charging, causing thermal runaway (thermorunaway) in which the temperature of the auxiliary battery becomes uncontrollable. Further, a lowest allowable voltage at and above which the auxiliary battery can be charged rises as the ambient temperature or the temperature of the auxiliary battery itself lowers, and where charging is performed at a voltage lower than the lowest allowable voltage, the auxiliary battery fails to be charged, causing insufficient charging.
Patent Document 1 discloses a power supply system for an electric vehicle, in which the temperature of the auxiliary battery is detected by a temperature sensor or the like and the charging voltage is set variably in accordance with the temperature of the auxiliary battery.
The use of a temperature sensor for detecting the temperature of the auxiliary battery is, however, undesirable because it leads to increase of cost.
In Patent Document 2, the state of the auxiliary battery (low-voltage battery) is determined and the charging voltage is set on the basis of the engine cooling water temperature detected by an engine cooling water temperature sensor and the outside air temperature detected by an outside air temperature sensor, instead of using a temperature sensor for detecting the temperature of the auxiliary battery, thereby restraining increase of cost due to the use of an additional temperature sensor.